1. Field of the Invention
The present invention relates to a rotary anode type X-ray tube and a method of manufacturing the same.
2. Description of the Related Art
As known to the art, a rotary anode type X-ray tube comprises a rotary structure having a bearing section. The rotary structure is rotatably supported by a stationary structure. Also, a disk-like anode target is fixed to the rotary structure. In an X-ray tube of this construction, an electromagnetic coil of a stator arranged outside a vacuum vessel is energized so as to rotate the rotor fixed to the rotary structure. As a result, the anode target is rotated at a high speed together with the rotary structure. Under this condition, an electron beam emitted from a cathode is allowed to strike against the anode target rotating at a high speed so as to cause an X-ray emission.
The bearing section is formed of a roll bearing such as a ball bearing or a dynamic pressure type slide bearing utilizing a spiral groove formed in the bearing surface and a liquid metal lubricant filling a bearing gap, i.e., a gap between the outer surface of the stationary structure and the inner surface of the rotary structure. The liquid metal lubricant includes, for example, gallium (Ga) and a gallium-indium-tin (Ga-In-Sn) alloy. The rotary anode type X-ray tube comprising a dynamic pressure type slide bearing is exemplified in, for example, Japanese Patent Publication (Kokoku) No. 60-21463 (which corresponds to U.S. Pat. No. 4,210,371), Japanese Patent Disclosure (Kokai) No. 60-97536 (which corresponds to U.S. Pat. No. 4,562,587), Japanese Patent Disclosure No. 60-117531 (which corresponds to U.S. Pat. No. 4,641,332), Japanese Patent Disclosure No. 62-287555 (which corresponds to U.S. Pat. No. 4,856,039), Japanese Patent Disclosure No. 2-227948 (which corresponds to U.S. Pat. No. 5,068,885), Japanese Patent Disclosure No. 2-244545 (which corresponds to U.S. Pat. No. 5,077,776) and Japanese Patent Disclosure No. 2-227948 (which corresponds to U.S. Pat. No. 5,068,885).
In the rotary anode type X-ray tube disclosed in the prior art documents exemplified above, a fine bearing gap sized about, for example, 20 .mu.m is provided in the dynamic pressure type slide bearing section having a spiral groove. These spiral groove and the bearing gap are filled with a liquid metal lubricant. Naturally, the lubricant is required to permeate over the entire region of the bearing gap in order to obtain a sufficient dynamic pressure for the slide bearing and, thus, to maintain a stable operation of the dynamic pressure type slide bearing. Where the lubricant fails to permeate over the entire region of the bearing gap, collision takes place between the outer surface of the stationary structure and the inner surface of the rotary structure in the worst case, with the result that the rotary structure is made incapable of rotation or is broken. To prevent such a problem, a lubricant chamber communicating with the bearing section is formed so as to ensure supply of a sufficient amount of a liquid metal lubricant to the bearing section even where the X-ray tube is operated over a long period of time.
In assembling the X-ray tube, a gas must be released completely from within the members constituting the bearing and from the lubricant. If the gas fails to be released sufficiently, the liquid metal lubricant is blown outside together with bubbles of the gas from the slide bearing section so as to be scattered within a vacuum vessel. In this case, the slide bearing fails to perform a stable dynamic pressure bearing function over a long period of time. Further, the liquid metal lubricant scattered within the vacuum vessel of the X-ray tube brings about a decisive defect that the withstand voltage of the apparatus is markedly impaired.